Do you compare only the responses of the same person at different times

Yes. There's too much variation between persons. I also think there's variation between types of pain and variation depending on whether there are other symptoms. There are no objective specific referents but people who are in actual serious pain usually look like it, are tachycardic, hypertensive, aggressive, sweating, writhing or very still depending on what type of pain were talking about. Real pain is also aggravated by relevant manual examinations.